Copper (Cu) metallization is gaining momentum in replacing aluminum (Al), particularly for the 0.18 um technology node and beyond. Due to the difficulty in dry etching Cu, a damascene approach is widely used for Cu metallization. This requires the Cu metallization process to have a high gap fill capability. The sputtering process widely used for Al metallization is not applicable to Cu metallization due to its inherent limitation in step coverage. Chemical vapor deposition (CVD) used in tungsten (W) metallization is not preferred for Cu at this time due to issues with morphology, adhesion and the conformal nature (seam formation issue) of CVD Cu films. Currently, the only manufacturable process for depositing Cu for interconnect applications is electrochemical deposition (ECD), thanks to its bottom-up fill capability.
Electrochemical deposition (ECD) is a process to produce solid phase product (such as thin films) by electrochemical reactions. Cu ECD is a process to make Cu thin film through electrochemical reduction of Cu ion, represented by the following electrochemical equation: Cu+++2e−→Cu where e− represents electron. In order for ECD process to proceed, a copper seed layer is required to pass current and to serve as a nucleation layer.
After ECD copper fill (overfill), chemical-mechanical polishing is used to remove excess copper. Unfortunately, copper protrusion 14 normally occurs in dense feature areas during ECD, as shown in FIG. 1. This is due to an accumulation of accelerating species on top of small features. The copper protrusion requires longer overpolish time and otherwise increases the difficultly of a CMP process. Longer overpolish time is one of the major factors for increased sheet resistance. As device features continue to shrink and density continues to increase, the protrusion problem becomes more and more significant.